A number of types of diamond crown bits for core drilling of exploratory wells are known.
One is a diamond crown bit comprising a body and a diamond impregnated matrix which is held to the body by a weld-on layer and subdivided into a number of sectors or teeth by mud discharge ports. The ratio of the sector length to the length of the mud discharge port is equal to 5:1.
The aforesaid heretofore-known drilling bit suffers from low drilling efficiency with respect to hard low-abrasive rock due to the fact that the diamonds are liable to glaze on account of the cooling effected when the drilling sludge gets compressed under the bit end face, this being accounted for by too great a length of the bit sectors.
Another diamond crown bit is known (cf. USSR Inventor's Certificate No. 594,291, Cl. E 21 B 10/48, "Bulletin of inventions and discoveries in the USSR", 1978, No. 7, p. 137 (in Russian). It also has a body and a sector-type matrix held to the body by a weld-on layer for mud discharge ports between the sectors. Higher drilling efficiency is attained in this case due to shorter sectors of the matrix, the ratio of sector length to the mud discharge port length being 3:1.
The disadvantage inherent in said known diamond crown bit resides in reduced strength of attachment of the matrix sectors to the bit body and the resultant breakage and chippage of the sectors in the course of drilling.
One more diamond crown bit is known (cf. U.S. Pat. No. 2,818,233, Cl. E 21 B, published Dec. 31, 1957) for use in core drilling of hard rock, the construction of said bit incorporating a body and a diamond-impregnated toothed matrix. The teeth are reinforced with diamonds having an inclined cutting edge on an inclined end face for the mud discharge ports.
The aforesaid drilling bit is disadvantageous in having too low a service life. The service life is limited by the fact that efficient drilling can be carried out as only until the diamond cutting edges become wornout, which rules out self-sharpening of diamonds. Another disadvantage of said drilling bits is too low tooth endurance.
Still one more diamond crown bit is known (cf. USSR Inventor's Certificate No. 448,266, Cl. E 21 B 10/48, "Bulletin of inventions and discoveries in the USSR", 1974, No. 40, p. 61) to comprise a body carrying a matrix on one end thereof. The matrix is subdivided, by means of mud discharge ports, into a number of working sectors, each comprising a diamond-impregnated layer on a weld-on layer, as well as a spring-loaded insert adjacent to said layers made of an easily destructible abrasive material. In use. the insert of easily destructible material wears to form, together with the drilling cuttings, form an abrasive layer on the hole bottom which exerts a destructive effect upon the matrix. Thus, optimum exposure of diamonds from the bulk of the matrix is attained, which contributes to more efficient operation of the tool.
The aforesaid known drilling bit is disadvantageous in having a sophisticated construction and inadequately reliable holding of the sectors to the matrix, which is accounted for by the considerable height and small length of the sectors.